A PON (Passive Optical Network) system provides a high-speed and large-capacity optical access system in which multiple ONUs (Optical Network Units) share an optical fiber or an OLT (Optical Line Termination). The system supports the FTTH service as a technology capable of reducing costs per user. In the PON system, a downstream signal from OLT to ONU is equivalent to a continuous signal. An upstream signal from ONU to OLT is equivalent to a burst signal because the OLT uses time-division multiplexing for preventing collision. The OLT receives different levels of optical signals because the distance between the OLT and the ONU depends on users. Accordingly, the OLT needs to use a receiver that has high reception sensitivity and is capable of achieving a wide dynamic range.
The PON system is expected to provide more high-speed services. At the initial stage of a high-speed service, however, all existing users do not need the service. There may be a situation where existing bit-rate ONUs and high bit-rate ONUs exist mixedly. In such a situation, there is a need to control operation costs of telecommunications carriers and promote smooth migration from the existing system. To solve this problem, the OLT needs to use a multirate burst mode receiver, a capability of receiving burst signals at multiple bit rates. The IEEE 802.3av task force standardized 10G-EPON, a PON system at the transmission rate of 10.3125 Gbps. With regard to 10G-EPON, IEE Std 802.3av-2009 defines physical layer specifications for burst mode receivers compliant with burst signals at transmission rates of 10.3125 Gbps and 1.25 Gbps so that the OLT can concurrently accommodate OLTs compatible with GE-PON as an existing system.
Generally, the burst mode receiver includes a SerDes (Serializer/Deserializer) circuit that performs clock synchronization with a photoelectrically converted burst signal and converts a serial signal into a parallel signal. Let us consider a situation where the OLT receives burst signals at multiple bit rates. When the OLT receives a bit-rate signal incompatible with the SerDes circuit, an incorrect reception signal and clock is transmitted to a higher layer to cause malfunction. To solve this problem, the multirate burst mode receiver requires a function to control output to the SerDes circuit in accordance with signal transmission rates or a function to control SerDes-compatible bit rates.
In the PON system, the distance between the OLT and the ONU depends on users. The OLT receives optical signals at different levels. Accordingly, the burst mode receiver needs to have high reception sensitivity and a wide dynamic range. When burst signals are received at multiple bit rates, an optimum gain and band of an amplifier depends on the bit rate of a reception signal. Therefore, the amplifier setting needs to vary with bit rates so that the multirate burst mode receiver ensures high sensitivity.
Several technologies have been proposed with respect to multirate receivers. According to the technology described in K. Hara, et al. “Burst-mode Bit-rate Discrimination Circuit for 1.25/10.3-Gbit/s Dual-rate PON Systems” OFC2009, a TIA (TransImpedance Amplifier) output is split. Limit amplifiers corresponding to low and high bit rates amplify signals. The amplified signal is synthesized with a preamble pattern (contiguous 1s and 0s) and a 1-bit delayed signal. The bit rate discrimination is performed on the synthesized signal to control an input to the SerDes circuit.
The technology disclosed in JP-A-2007-243285 supports multiple bit rates by switching between an amplifier gain and a SerDes reference clock based on a control signal detected in the MAC layer based on an ONU transmission time slot. According to the technology disclosed in JP-A-2005-348047, a CDR (Clock Data Recovery) for reproducing a clock signal in the SerDes circuit compares the clock of an input signal with its own reference clock to discriminate a bit rate of the input signal and selects a subsequent signal processing circuit.